Exactly 12 months ago, on the last Sunday of December 2004, we were beginning to confront the reality of one of the most terrible natural disasters in living memory. It's impossible not to be aware of that anniversary: dates are etched in our minds. Our lives aren't just featureless strips of things going on.

We make maps of our lives, we tell stories divided by events where something happened to change things. For those most directly involved, the date of December 26, 2004, marks a brutal interruption - the death or injury of someone, terrible anxiety, bereavement, anger and bewilderment. But for all of us, the date will carry significance, for all of us something erupted into our comfortable consciousness.

Like 11 September and, now, 7 July as well, it stands in the landscape or the map, a feature that will never be obliterated. That was when things changed.

Anniversaries are among the things we most take for granted, in our personal lives - birthdays and wedding anniversaries - and in public life - 5 November, Remembrance Day. This last year, we have been marking 200 years since Trafalgar and 60 years since the end of the second war.

It's all the more strange, then, that so many are so reluctant to treat Christmas as an anniversary. Just as in the Millennium year, there was embarrassment about what it actually commemorated, so there is the same embarrassment about the event that Christmas marks. Yet again, we've had the reports of people trying to find ways of turning Christmas into a bland and empty winter jollification. The message seems so often to be, 'Don't remember the story; what matters isn't the real history of the world but just the cycle of the seasons. You only need to remember last year - it got cold and dark and then it started getting warmer and lighter again'.

How very bizarre that the most enlightened and progressive thinking of the Western world should take us back to the mindset of the cavemen!

All right; but why remember the story exactly?
Just as a heart-warming tale of a vulnerable baby? There are plenty of other stories about that.
What was it that changed when this particular baby was born in Bethlehem?
Why is it a vital part of the story of the whole world?

Christians have a quite elaborate answer, in terms of how this was the moment when God began to live as a human being, began to live the life that led to his redeeming death and transforming resurrection. But just for a minute, put this on hold. If you were an unbeliever or half-believer, what might convince you that this did indeed mark a change so significant that we'd still be thinking about it after 2,000 years? 'Everything necessary has been given us in the Gospels. What is it?

'Firstly, the love of one's neighbour - the supreme form of living energy. Once it fills the heart of man it has to overflow and spend itself.

'And secondly, the two concepts which are the main part of the make-up of modern man - without them he is inconceivable - the ideas of free personality and of life regarded as sacrifice.'

Words from one of the great novels of the 20th Century, a novel born out of the nightmare conditions of modern totalitarianism - Pasternak's Dr Zhivago. Again and again in this astonishing work, Pasternak returns to the point, the point of vision which gave him his own personal resource to fight back against the pressure to silence and conformity in Stalin's Russia.

'Something in the world had been changed. Rome was at an end. The reign of numbers was at an end... The story of a human life became the life story of God and filled the universe'.

What has changed? 'Rome was at an end' says Pasternak's character.

Christ was born into a society we can hardly imagine - though a somewhat lurid television series this autumn has captured some of it - in which any notion of the sanctity of every life was completely alien; some were born only to die - handicapped children, girl children in some places, exposed on hillsides to starve or freeze; slaves who existed to serve every passing desire of their masters and mistresses; outsiders, foreigners, who were not really human; gladiators whose job it was to kill or be killed for public amusement. It's not - let's be clear - that human behaviour has improved so spectacularly since the first Christmas that we can look back on these atrocities with complacency.

A country with our current rates of abortion cannot afford to rest on its ethical laurels; there is effective slavery among the poorest of our world; civilised societies have started flirting once again with the idea that torture might be acceptable. It isn't that we have left Roman-style inhumanity entirely behind; what has changed is that no-one now could possibly take these things for granted without coming up against a challenge from most of the main imaginative and moral currents of our European and Middle Eastern cultural history.

In other words, you may or may not believe what Christian doctrine says about the child in the manger; but you will, consciously or not, be looking at the human world in a framework that Jesus Christ made possible - which is, incidentally, quite a good reason for thinking twice before rejecting the doctrine.

A vision has been introduced into the world that cannot be expelled. We talk boldly - and for the most part rightly - about how we can't turn the clock back with ideals of democracy and accountability and freedom of conscience. No-one can pretend they haven't been thought about and in some degree realised among us.

True; but what about the fact that, ultimately, made all of them possible, the fact that put an end to Rome, to the age of unquestioned, inhuman empires and mass deaths that gave no-one any sleepless nights?

Sometimes today you can just faintly hear voices whispering that perhaps there was something to be said for the ancient world; that universal human dignity and the absolute wrongness of certain acts of violence and cruelty are nice ideas but a bit difficult in a complicated world. God forgive us. But if we do ever come to forget not just the Christmas story but what it made possible, the end of Rome, the arrival of a different humanity, there is enough, sadly, in our idle and self-obsessed hearts to let the ancient world begin to creep back a little bit more. I don't believe that in fact it could be possible to forget. When modern tyrannies have tried to make people forget, memory has shown itself pretty tenacious, secretly, obstinately, subversively.

After all, if it's simply true that Jesus was born and lived and died and rose as he did, things just have changed; you can deny that the sun has risen if you like, but only by insisting on keeping your eyes tight shut. All around, the landscape has changed, and people are discovering that they are capable of living differently in the company of Jesus.

A few weeks ago, Gee Walker, mother of the murdered Liverpool teenager, Anthony Walker, told us that yes, she forgave her son's killers and yes, her heart was still broken. What made this so intensely moving was the fact that her forgiveness was drawn agonisingly out of her, without making her loss easier. She could not have been who she was if she did not recognise that forgiveness was laid upon her; her life and her dead son's would have been nonsense if she did not forgive.

It was mercy without a hint of trivialisation or excuse for wrongdoing. No preacher could say it like that, could make it sound utterly true and costly and necessary all at once. And last week, the mother of Abigail Witchalls, paralysed by a knife attack in April, described her sadness about Abigail's attacker, who had killed himself.

'His death is the real tragedy in this story', she wrote, not making light of her daughter's terrible ordeal or denying the complex evil of the action, but simply making space in her heart for someone else's fear and pain. Why remember what happened at Bethlehem, why resist the efforts to reduce it to a brief fling of sentimental goodwill in the middle of bad weather?

Because of people like these. They have known in their flesh and nerves just what the difference is that Jesus makes; it is not comfort or easy answers, it is the sheer fact that - we have to use the word - miraculous love is possible. The vilest offender, as the hymn says, is now deserving of attention and compassion; no life can be allowed to fall out of the circle of love.

Because God has overthrown the empire of numbers and calculations, mass movements and majority interests: 'The story of a human life became the life story of God and filled the universe.'

Remember this day; this was when the new creation began.

Dr Rowan Williams is the Archbishop of Canterbury. This is his 2005 Christmas sermon.